Conventionally, such an image formation apparatus has been known widely that forms an image on a transfer medium such as a transfer film and an image carrying body or a recording medium such as a card and sheet. This type of image formation apparatus uses an indirect printing scheme for forming an image (mirror image) on a transfer medium using an ink ribbon, and next transferring the image formed on the transfer medium to a recording medium, or a direct printing scheme for forming an image directly on a recording medium using an ink ribbon.
Such an image formation apparatus is generally provided with an image formation section having a thermal head with a plurality of heating elements arranged, and platen (for example, platen roller) disposed opposite the thermal head, transports an ink ribbon and a medium (transfer medium in the indirect printing scheme, recording medium in the direct printing scheme) at the same velocity while supporting the back side (opposite surface side to the image formation surface) of the medium by the platen, selectively operates (heat-controls) the heating elements of the thermal head brought into press-contact with the ink ribbon, and thereby forms an image.
Further, in such an image formation apparatus, there is the case where printing is performed with single-color ink (for example, Bk (Black) ink), and there are many cases that color printing for generating a color image is performed by superimposing respective images with inks of a plurality of colors. In the case of color printing, printing is performed by superimposing images for each of inks of a plurality of colors (for example, inks of YMC) on a medium, according to input printing data or printing data (for example, printing data for each of Y (Yellow), M (Magenta) and C (Cyan)) obtained by converting input image data. Therefore, in color printing is used an ink ribbon with ink panels of a plurality of colors and a Bk (Black) ink panel as necessary repeated in a face sequential manner. In addition, for example, the Bk ink is used in the case of sharpening the contour, or in the case of forming an image of logo, character and the like.
In addition, in the image formation apparatus using the ink ribbon, a phenomenon for degrading printing quality on the medium arises by effect of a wrinkle occurring in the ink ribbon. This wrinkle occurs due to a level difference in a rear end corner portion in the ink ribbon transport direction of a printing region of the ink ribbon. Usually, in the case where tension on the ink ribbon is uniformly applied, any wrinkle does not occur even when such a level difference arises. However, when the tension balance is lost due to some cause, the wrinkle caused by the level difference occurs easily.
The phenomenon will be described more specifically. As shown in FIGS. 17A and 17B, for example, wrinkles Wr of an ink ribbon 41 are caused by a level difference due to ink removal in rear end corner portions in the transport direction of a printing region Rt of a Y ink panel of the ink ribbon 41 in image formation, and spread to a printing region Rt of a next M ink panel. Therefore, when an image of M ink is formed on the medium using the printing region Rt of the M ink panel with the wrinkles Wr formed, by effects of the wrinkles Wr extended from the Y ink panel, on the medium occurs pixel removal (a part of pixels are not printed) and pixel hopping (a part of pixels are printed in a position different from a position indicated by printing data.) In color printing, since printing is performed by superimposing images for each of inks of a plurality of colors, when the pixel removal and/or pixel hopping occurs, printing quality degrades.
On the other hand, conventionally, on the transfer medium, known are techniques (for example, see Patent Document 1) for arranging a dummy image so as to enclose a printing image, and techniques (for example, see Patent Document 2) for arranging dummy images on the opposite outer sides of the printing image.